docs: def _verify_include_files_used(self, file_uses, included_files):
          """Find all #include files that are unnecessary."""
          for include_file, use in file_uses.items():
              if not use & USES_DECLARATION:
                  node, module = included_files[include_file]
                  if module.ast_list is not None:
                      msg = "'{}' does not need to be #included".format(
                          node.filename)
                      if use & USES_REFERENCE:
                          msg += '; use a forward declaration instead'
                      self._add_warning(msg, node)